In recent years, Doherty amplifiers that achieve highly efficient operation have been proposed as amplifiers for communication.
A Doherty amplifier includes a carrier amplifier and a peaking amplifier connected in parallel, and a 90-degree line for performing load modulation loaded at the output of the carrier amplifier, to achieve highly efficient operation during back-off operation in which output power is lower than saturation power. The back-off operation refers to a state in which the peaking amplifier is not operating.
In the Doherty amplifier, however, the electrical length of the 90-degree line may deviate, in principle of operation, from 90 degrees owing to the frequency dependence of the 90-degree line loaded at the output of the carrier amplifier, thus resulting in narrow bandwidth of the operation.
Patent Literature 1 mentioned below discloses a Doherty amplifier including a plurality of lines having different electrical lengths at the output of a carrier amplifier, and including a control mechanism for selecting a line to be connected to the output of the carrier amplifier from the plurality of lines having different electrical lengths according to the frequency of an input signal.
The control mechanism serves to make the electrical length of a line at the output of the carrier amplifier closer to 90 degrees.